Constructing protective forests to control water and soil erosion is an effective measure to address land degradation in the Bashang Plateau of North China, but forest dieback has occurred frequently due to severe water deficits in recent decades. However, transpiration dynamics and their biophysical control factors under various soil water contents for different forest functional types are still unknown. Here, canopy transpiration and stomatal conductance of a 38-year-old Ulmus pumila L. and a 20-year-old Caragana korshinskii Kom. were quantified using the sap flow method, while simultaneously monitoring the meteorological and soil water content. The results showed that canopy transpiration averaged 0.55 ± 0.34 mm d−1 and 0.66 ± 0.32 mm d−1 for U. pumila, and was 0.74 ± 0.26 mm d−1 and 0.77 ± 0.24 mm d−1 for C. korshinskii in 2020 and 2021, respectively. The sensitivity of canopy transpiration to vapor pressure deficit (VPD) decreased as soil water stress increased for both species, indicating that the transpiration process is significantly affected by soil drought. Additionally, canopy stomatal conductance averaged 1.03 ± 0.91 mm s−1 and 1.34 ± 1.22 mm s−1 for U. pumila, and was 1.46 ± 0.90 mm s−1 and 1.51 ± 1.06 mm s−1 for C. korshinskii in 2020 and 2021, respectively. The low values of the decoupling coefficient (Ω) showed that canopy and atmosphere were well coupled for both species. Stomatal sensitivity to VPD decreased with decreasing soil water content, indicating that both U. pumila and C. korshinskii maintained a water-saving strategy under the stressed water conditions. Our results enable better understanding of transpiration dynamics and water-use strategies of different forest functional types in the Bashang Plateau, which will provide important insights for planted forests management and ecosystem stability under future climate changes.

The Bashang Plateau is the core zone of the agro-pastoral ecotone in northern China and represents an ecological barrier for preventing the invasion of wind-blown sand in the Beijing–Tianjin–Hebei region. Increasing plant cover to control soil erosion is an effective measure to address land degradation; however, plant cover is different from climatic conditions. In this study, we determined the optimal spatial distribution of Populus simonii Carr., which is a widely planted species used for revegetation on the Bashang Plateau. A modified Biome-BGC model was used to simulate the dynamics of the net primary productivity (NPP), actual evapotranspiration (AET), and leaf-area index (LAI). The model was validated using field-observed tree-ring and MODIS AET and NPP data. The dynamics of AET, NPP and LAI for P. simonii at 122 representative sites in the study area for the period 1980–2019 were simulated by the validated model. The results showed that the spatial distributions of mean AET, NPP, and LAI generally decreased from southeast to northwest. The ranges of optimal plant cover in terms of maximum LAI for P. simonii were 3.3 in the Fengning–Weichang area, 1.9 in the Shangyi–Zhangbei–Guyuan area and 1.3 in the Kangbao area. Mean annual precipitation (MAP), elevation, soil texture and mean annual temperature were the main factors influencing the distribution of AET, NPP and LAI. As the MAP decreased, the correlations between AET, NPP, LAI and precipitation gradually decreased. In different subregions, the factors influencing optimal-plant-cover distribution varied significantly. These quantitative findings provide the optimal plant cover for the dominant tree in different subregions and provide useful information for land degradation management on the Bashang Plateau.